U.S. Pat. No. 5,821,972 describes an electrographic printing apparatus which includes a developer supply for supplying a developer having a toner component; a print head for transferring toner from the developer supply in an image wise manner; and a compliant receiver for receiving the image wise toner from the print head. The receiver has a compliant inner conductive blanket layer for allowing the receiver to conform to a print medium and a non-compliant overcoat layer for efficiently releasing toner from the receiver. The image wise toner is transferred from the compliant receiver to the print medium at a transfer station.
U.S. Pat. No. 5,689,787 describes forming a small particle toner image on a primary image member, such as a photoconductor; electrostatically transferring the image to an intermediate transfer member; and then electrostatically transferring the image to a receiving sheet. The intermediate transfer member includes a substrate, a compliant blanket, and a thin, hard overcoat sectioned into small, discreet segments.
U.S. Pat. No. 5,835,832 (1998) teaches an improved method and apparatus for robust transfer of toner images using toner particles having a volume average diameter between about 2 μm and about 9 μm. Surprisingly good electrostatic transfer is obtained when the surface charge density of the toner is between 3.0×10−9 coul/cm3 and 6.5×10−9 coul/cm3 and when this toner is used in conjunction with a compliant transfer intermediate.
U.S. Pat. Nos. 5,728,496 and 5,807,651 describe unexpectedly good transfer of electrophotographically-produced images using small toner particles when the image is developed on an electrostatographic recording member, preferably an organic photoconductive element, which has been overcoated with a thin (about 10 nm to about 10 μm thick) layer of a material having a Young's modulus greater than 10 GPa and preferably greater than about 100 GPa. The image is then transferred to an intermediate member which is comprised of an elastomeric blanket between about 0.1 and about 3 cm thick, having a Young's modulus between about 0.5 MPa and about 50 MPa, and preferably between about 1 and about 10 MPa, and having an electrical resistivity between about 106 ohm-cm and about 1012 ohm-cm, by applying an appropriate electrostatic potential between the transfer intermediate member and the photoconductive element. The toned image is transferred from the intermediate transfer member to the receiver by applying an electrostatic field between the receiver and the intermediate transfer member. The blanket material comprising the intermediate transfer member should be overcoated with a thin (between about 0.1 μm and about 25 μm thick) layer of a material having a Young's modulus greater than about 100 MPa and preferably greater than about 1 GPa.
In an electrostatographic engine such as an electrophotographic engine, an electrostatic latent image is initially formed on a primary imaging member such as a photoreceptor and then developed into a visible image using marking particles, often referred to as toner or dry ink particles. This image is then transferred to a receiver such as paper, generally upon application of an electrostatic field that urges the electrically charged toner particles towards the receiver and the image is then permanently fixed by passing the image-bearing receiver through a fuser that melts the toner particles and permanently fixes them to the receiver. The receiver is transported through the electrophotographic engine using a receiver transport mechanism as is known in the art.
Color images are generally produced by first producing separate electrostatic latent images corresponding to the cyan, magenta, yellow, and black information, toning each of these separations with toner consisting of the correct primary subtractive toner, and then superimposing these separate images on the receiver. Images comprising principally a certain color (e.g. black alphanumeric characters) with a certain localized color such as a corporate logo, could also produce images in a similar manner. In this latter example, however, toners corresponding specifically to those used in the image, rather than process colors comprising the subtractive primary colored toners, are generally used. Transfer is often accomplished by wrapping the receiver around an electrically biasable transfer roller and sequentially transferring the separations, in register, to the receiver by applying an appropriate electrical bias to the transfer roller.
Under certain circumstances, it is advantageous to transfer the toned image first to an intermediate transfer member and then from that intermediate transfer member to the receiver. For example, by transferring the toned color separations to the intermediate, the receiver need not be picked up and wrapped around the transfer roller and then released after transfer. This allows the use of a straight paper path, simplifies the process, and reduces the probability of having a paper jam.
Of particular advantage to enhance the electrostatic transfer of toned images is the use of a compliant intermediate, as disclosed by Rimai et al. (U.S. Pat. No. 5,084,735), wherein a multilayer transfer intermediate comprising a compliant layer having a Young's modulus of 107 Pa or less and a thin outer skin having a Young's modulus of 5×107 Pa or greater. The advantage of a compliant intermediate over noncompliant intermediates is that it facilitates the transfer of toner particles by allowing the toner particles to contact both the primary imaging member and the receiver, in a manner similar to that disclosed by Rimai and Chowdry in U.S. Pat. No. 4,737,433. It should be noted that U.S. Pat. No. 4,737,433 teaches the use of monodisperse toner particles and very smooth receivers and, therefore, does not directly read on the present invention.
In U.S. Pat. No. 5,370,961, Zaretsky and Gomes disclose the transfer of toner particles having a mean particle diameter of less than 7 μm, said toner particles comprising transfer-assisting particles strongly adhering to the surface of the toner particles, said transfer-assisting particles having a mean particle diameter between 0.01 and 0.2 μm from a compliant intermediate such as that disclosed by Rimai in U.S. Pat. No. 5,084,735, but also restricting that compliant intermediate to one whose average surface roughness is equal to or less than 20% of the mean toner diameter.
Ezenyilimba et al, in U.S. Pat. No. 5,968,656, disclose an outer surface network comprising the cross-linked reaction product between a polyurethane with reactive alkoxysilane moieties and tetraalkoxysilane, hereafter referred to as a ceramer and incorporated by reference into the present disclosure. According to that disclosure, the silicon oxide network comprises between 10 and 80% of the ceramer, preferably between 25 and 65% of the ceramer, and more preferably between 35-50% of the ceramer. Moreover, it is important that the ceramer have a storage modulus between 0.10 and 2.0 GPa and preferably between 0.30 and 1.75 GPa, and more preferably between 1.0 and 1.5 GPa. Ezenyilimba et al. does not teach the use of a compliant intermediate comprising a ceramer with toner particles comprising transfer-assisting particles. In certain electrostatographic or electrophotographic engines (hereafter referred to simply as electrophotographic engines unless otherwise denoted), the primary imaging member is driven by the intermediate transfer member. The intermediate may be directly driven by a motor or other suitable means. Alternatively, the intermediate transfer member may be driven by another member such as the receiver transport member or web. In either case, the use of an intermediate transfer member as a drive member will cause stresses in that member as a result of the torques required to drive the other members. This is especially problematic with compliant intermediates, wherein the relatively low Young's moduli of compliant intermediates, will result in relatively large strains. Such strains can readily cause the overcoat layer of the compliant blanket of the intermediate to crack and craze. These cracks can widen as a result of the stresses resulting from the use of the intermediate transfer member as a drive member, thereby creating image artifacts in the transferred image. Moreover, the occurrence of cracks can cause the overcoat layer to delaminate from the underlying elastomeric blanket, thereby making the roller too adhesive and consequently adversely affecting transfer. As is well known, materials with relatively high Young's moduli tend to crack under lower strain conditions than do materials with lower Young's moduli. Accordingly, the use of a relatively high modulus overcoat, such as those disclosed in the patents by Rimai et al., Zaretsky et al., and Ezenyilimba et al. may not function at all in an electrophotographic engine in which a compliant intermediate member is used to drive another member such as a primary imaging member.
Another constraint on the values allowed for the Young's modulus of the overcoat layer of a compliant intermediate arises from the use of transfer-assisting particles appended to the surface of the toner particles. Specifically, in an ideal world of spherical particles, the force needed to detach a particle from a substrate is independent of the Young's modulus of that substrate. The force needed to detach a toner particle from an intermediate member has a direct bearing on one's ability to either transfer from the primary imaging member to the intermediate transfer member or from the intermediate transfer member to the receiver. In other words, if toner particles are held too strongly to the intermediate transfer member, it becomes difficult to transfer from that member to the receiver. Conversely, if the intermediate transfer member is not sufficiently adhesive, toner may not transfer to that member from the primary imaging member. In the nonideal world of irregularly-shaped toner particles, toner adhesion is often controlled by the interaction of the asperities on the toner with the underlying substrate. While it is not the intention to base the validity of this patent on an explanation of this phenomenon, such an explanation can help elucidate the underlying interactions giving rise to the present invention.
Toner particles interact with substrates such as compliant intermediates, primary imaging members, receivers, and the like by two types of forces. The first comprises long range electrostatic forces arising from the electrostatic charge on the toner particle. The second is the short range van der Waals interactions. Van der Waals interactions are generally significant at separation distances between two bodies of less than 10 nm and tend to increase linearly with the diameter of the particle. For particles the size of toner particles that are typically used today (between approximately 4 and 12 μm), experimental evidence suggests that the dominant mode of interactions arise from van der Waals forces. Accordingly, if a particle has asperities on its surface that separate the bulk of the particle from a substrate by a few nanometers, the force adhering the particle to the substrate would depend on the radius of the substrate, not on the radius of the particle. Accordingly, the role of the transfer-assisting particles that are appended to the surface of the toner particles is to physically separate the toner particles from the underlying substrate such as a primary imaging member or a transfer intermediate member, thereby facilitating transfer of the toner particles from one member to another under the influence of the applied electrostatic transfer field. Ideally, the transfer-assisting particles should have diameters close to approximately 10 nm, which would minimize the force of adhesion between that particle and the contacting substrate without significantly contributing van der Waals forces of adhesion of its own. In reality, the transfer-assisting particles are often somewhat larger, typically between approximately 30 nm and 50 nm. It should be noted that the stated size of the transfer-assisting particles is often the diameter of agglomerates of smaller fundamental particles, but that distinction is not important for this invention.
A difficulty arises when using toner particles comprising transfer-assisting particles appended to the surface of the toner particles with compliant intermediates. If the underlying substrate deforms sufficiently under the stresses associated with the forces of adhesion (including electrostatic) or the applied pressures existing in the transfer nip, the toner particles may become sufficiently engulfed into a compliant intermediate, notwithstanding the presence of the overcoat, so as to totally engulf the transfer assisting particles and thereby negating any influence on transfer that they may have had. At the lower range of values of the Young's modulus of the overcoat, as disclosed in the related art, this can readily occur, as will be shown in this disclosure. Indeed, Zaretsky et al. had to require that the surface of the intermediate transfer member be smooth to minimize this problem. Requiring such smoothness is often difficult in a real-life manufacturing process. Moreover, Zaretsky et al. also allowed the transfer-assisting particles to have diameters as great as 200 nm. As previously discussed, as the size of the transfer-assisting particles increases, their contribution to the adhesion of the toner particles also increases.
It is not obvious from the related art that a compliant intermediate that is capable of transferring toner particles between 4.0 μm and 10.0 μm that can also be used to drive another member or members of an electrophotographic engine could be produced. More specifically, there is no range of values of the Young's modulus of the overcoat, when used with toner particles comprising transfer-assisting particles between approximately 20 nm and 70 nm appended to the surface of the toner particles, could be used also as a drive mechanism in an electrophotographic engine.